The present invention is directed to a method and apparatus for purifying aqueous liquids containing particulate matter and to a filter element, assembly, and method for effecting such purification. More specifically, the present invention relates to the treatment of aqueous liquids employed in power plants, more particularly nuclear power plants, to reduce the amount of insoluble iron present in such aqueous liquids.
Techniques, materials and devices for separating particulate matter from fluids have existed for centuries. Many such separations involve relatively low-level technology and simple materials. As science and technology have advanced, however, new materials and techniques have permitted separations to be achieved to meet the requirements for ever purer materials. Considering the developments in materials science in recent years, when viewed broadly, the separation of particulate matter from a fluid might seem to be a simple task. However, many such separations remain unresolved or, more typically, resolved only to the extent that the results obtained fall short of the purification sought.
Some of the factors which have resulted in less than complete separation include the large volumes of fluids being processed, the type of filtration media available for such separation processes, the nature and chemical composition of the particulate matter being removed, the fineness of the particulate matter and the nature of the fluids in which the particulate matter is found.
To illustrate some of the problems which result from such factors, one could consider any large scale industrial process in which large volumes of liquid are employed. The adverse effects of particulate matter present in the liquids being employed will vary from one process to another. Thus, while particulate matter may be tolerated in any amounts in certain processes, other processes require total elimination of particulate matter. Likewise, the particle size of such solid matter may be of little or no significance in some processes but critical in others. Intermediate these extremes, the specifications of some processes permit certain amounts of solid particulate matter as long as the particles fall above or below predetermined sizes.
Techniques and filtration materials have ranged at the low technology end of the separation spectrum from simple sieves or beds of readily available materials to the other end of the spectrum where new media have been developed to achieve separations of particulate matter from fluids in which the physical and chemical natures of the fluids, particulate matter and/or filtration media are carefully selected to achieve separation. The present invention relates to the latter type of separation.
One area in which removal of fine particulate contaminants is a major concern, is in the field of electric power generation systems. In such systems, which may be fossil fuel powered or nuclear powered, high purity feedwater is heated in a boiler to create either pressurized high temperature water or steam which is then expanded through a steam turbine. The shaft of the turbine is connected to an electric generator shaft which, when rotated, creates electrical energy. The steam which exits from the turbine is condensed in a heat exchanger and, typically, is subsequently purified and reheated. The condensed water is then directed back to the boiler as feedwater, completing a power cycle.
Frequently, the electric power generating plants purify the condensate to remove contamination, particularly ionic materials and particulate matter, which may either be present in the raw water supply, or may enter the feedwater, steam, or condensate from a variety of sources during the power cycle. Ionic materials may be removed by the use of demineralizers, to purify condensate through an ion exchange technique. Two types of demineralizers are used for condensate purification, namely, deep bed demineralizers and filter demineralizers.
Deep bed demineralizers use resin beads to remove dissolved ions in the condensate. Specifically, the condensate is passed through a bed of resin beads which are retained in a demineralizer vessel. The deep bed demineralizers typically have an effective pore rating in the condensate water of about 40 to 50 microns and are only marginally useful in removing particulates from the condensate.
Filter demineralizers employ powdered ion exchange resins and/or inert filter aids such as cellulosic fibers which are precoated onto fine porous elements and are sometimes referred to as xe2x80x9cprecoatsxe2x80x9d. Such elements typically include spirally welded metal elements, powdered metal elements, wedge wire elements and yarn or string wound elements. The condensate is passed through the precoated elements which remove dissolved contaminants and trap particulates. The precoats on the filter demineralizers typically have an effective pore rating of about 5 to 30 microns, and the underlying filter media have a pore rating of about 5 to 120 microns. The filter demineralizers have an overall effective pore rating of about 5 to 30 microns and are, therefore, somewhat more effective in removing particulates from condensate, as compared to deep bed demineralizers. Under certain operating conditions, however, solids levels are relatively high and lead to the need for extensive backwashing of the precoat resin, with the concomitant high cost of operation of the filter demineralizers.
Contaminants in feedwater, steam and condensate in fossil fuel-powered generating plants typically must be maintained at a level of total suspended solids of no greater than about 50 to 250 parts per billion (ppb), and most typically no more than about 50 ppb total suspended solids. In nuclear power plants tolerances for solid particulate contaminants are frequently much lower, typically about 0.025 to about 2.0 ppb. Certain types of contaminants, such as iron-containing contaminants are tolerated even less in nuclear power plants, particularly of the Boiling Water Reactor (BWR) type.
The nuclear power industry has recognized that as an important first step in reducing radiation fields and occupational exposure, it is necessary to reduce iron input to the BWR primary system. For feedwater iron, the recognized optimum concentration is not more than about 0.1 to 0.5 ppb. Several techniques have been employed to reduce feedwater iron levels for optimized water chemistry using condensate filtration.
Typically, iron found in the reactor vessel enters by way of the feedwater system and deposits on fuel cladding surfaces where soluble reactor water impurities, such as cobalt, are absorbed. Subsequently, the absorbed metal impurities become neutron activated and a portion thereof are later released to the reactor water as soluble or insoluble radioactive isotopes. Thereafter, they are transported by reactor water throughout the primary system and accumulate on piping and equipment surfaces, resulting in increased dry well and reactor building general area dose rates. Thus, by reducing the concentration of iron in the primary system, the amounts of deposited iron and non-radioactive cobalt (59Co) absorbed by the iron is reduced, resulting in a reduced source of activated 60Co.
Although methods have been developed to limit radiation buildup on primary system surfaces, there has developed and still remains a strong need to limit feedwater iron concentrations. Thus, a technique has been employed recently in which zinc is injected into the feedwater to maintain a specified concentration of soluble zinc in the reactor water which thereby limits radiation buildup on primary piping components. In a modification of the technique which optimizes control of radiation buildup, a depleted zinc oxide method was developed to minimize the unwanted production of radioactive 65Zn from neutron activation of 64Zn. When the concentration of feedwater iron is high, however, the amount of depleted zinc oxide necessary to maintain the desired reactor water concentrations of zinc is increased due to the absorption of zinc by iron. As a result, high feedwater iron concentrations increase the amount of zinc necessary to produce the beneficial results. Accordingly, it is important to maintain low feedwater iron levels to reduce the cost associated with high amounts of depleted zinc oxide that are necessary to control radiation.
Reduced iron feedwater concentrations also benefit in the use of hydrogen water chemistry (HWC). This is a technique for imparting reducing characteristics to water by forming an aqueous solution of hydrogen in water.
Improved condensate filtration offers further advantages in addition to controlling radiation buildup and reducing occupational exposure. By further reducing the amount of insoluble iron entering deep bed demineralizers, the life and performance of the resins used therein can be substantially improved. Thus, reducing iron fouling, or coating of the resin beads with crud, will improve the ion exchange performance and substantially eliminate the need to periodically clean the resins by ultrasonic means or backwashing. In addition, since such cleaning methods may be virtually eliminated, the resin bed is not disturbed or mixed and the usable ion exchange capacity is increased by maintaining the chromatographic integrity of the resin column by maintaining resin beads with low ionic loading at the bed effluent. The effect of reducing ion concentrations introduced to deep bed demineralizers is expected to be improved feedwater quality and reduced liquid and solid radioactive waste. Furthermore, with improved ion exchange performance derived from prefiltration, plants which use chemical regeneration can reduce the frequency of regeneration, or possibly eliminate the necessity of regeneration entirely.
Heretofore, prefiltration of iron particulates from power generating plants has required frequent backwashing of the filter elements to avoid excessive pressure drop across the filter medium. This is troublesome from the standpoint of maintenance cost. Moreover, in the case of nuclear power plants, frequent backwashing requires diversion of a considerable quantity of radioactive condensate as backwash liquid which poses its own disposal problems. Often, the backwashing cycle of currently used prefilters may be as short as five days.
It is accordingly an aspect of the invention to filter fine particulate matter, for example, down to 0.5 xcexcm or even smaller.
It is another aspect of the invention to provide a method for treating condensate from a power generating plant, particularly a nuclear power plant, and most particularly a boiling water reactor nuclear power generating plant, which can efficiently and economically reduce the contaminant level of the condensate to the requisite level.
It is another aspect of the invention to provide a treatment method which can also be used to continuously purify or polish condensate from a power generating plant, particularly a nuclear power generating plant and most particularly a BWR nuclear power generating plant to ensure that the contaminant level of the condensate remains within an acceptably lower level.
It is a further aspect of the invention to provide a filtration element, filter assembly and a method of treating aqueous condensate from a power generating plant, particularly a nuclear power generating plant, and most particularly a BWR nuclear power generating plant, to reduce feedwater iron to a range of 0.1 to 0.5 ppb.
It is still another aspect of the invention to provide a purification method and separation element for treating condensate from a power generating plant, particularly a nuclear power generating plant, and most particularly a BWR nuclear power generating plant, to achieve minimum 21-day prefilter and 50-day filter/demineralizer cycles (the length of time backwashing operations for non-precoated and precoated filters, respectively) with 30 ppb inlet crud levels.
It is yet another aspect of the invention to provide a method of treating condensate from a power generating plant, particularly a nuclear power generating plant, and most particularly a BWR nuclear power generating plant, which requires less than 40,000 gallons per year of liquid radioactive waste to backwash the full flow system.
It is still another aspect of the invention to provide a purification element and a method which treats condensate from a power generating plant, particularly a nuclear power generating plant, and most particularly a BWR nuclear power generating plant, which achieves a filter life durability of at least two fuel cycles (i.e., the time between refueling) or three years.
It is yet another aspect of the invention to provide a purification method and a filter element which significantly reduce the frequency of backwashing required to provide continued effective filtration.
It is also an aspect of the invention to provide a purification method with a filter element having a high surface area, inexpensive filter membrane.
In accordance with one aspect of the invention, a method for separating iron-containing particulate or colloidal contaminants from an aqueous liquid comprises passing aqueous liquid including the iron-containing contaminants in one direction through a pleated filter element. Passing the aqueous liquid through the pleated filter element includes directing the aqueous liquid through a pleated hydrophilic film comprising an ultra-high molecular weight polyethylene and having a pore size in the range from about 0.001 micron to about 10 microns and reducing the iron-containing contaminants in the aqueous liquid to a level of about 2 parts per billion or less. The method further comprises cleaning the pleated filter element by directing a backwash fluid in an opposite direction through the pleated filter element, including directing the backwash fluid in the opposite direction through the pleated, hydrophilic, ultra-high molecular weight polyethylene film.
The present invention may be employed in power generating systems during startup and steady state operating conditions. The present invention reduces the concentration of solid particulate contaminants in aqueous liquids, particularly aqueous liquids employed in power plants to no more than about 2 ppb, preferably no more than about 1 ppb and most preferably no more than about 0.5 ppb of total suspended solids. The present invention also reduces iron-containing particulate contaminants in power plant condensates to a level of no more than about 2 ppb, preferably no more than about 1 ppb and most preferably no more than about 0.5 ppb. In addition, because of the nature of the filters and filtration systems employed, in most situations infrequent or no backflushing or backwashing is required for prolonged use of the filters, although such backwashing is not proscribed.
The present invention includes a method of treating condensate polishing in the environment of a power plant, particularly a nuclear power plant, and most particularly a BWR nuclear power plant, which includes passing the contaminated condensate through a hydrophilic filter medium so as to reduce the total suspended solids of the condensate to no more than about 0.5 ppb. Preferably, the filter medium employed in the present invention has a removal efficiency of about 99.98% at no more than about 1.5 microns.
When the filter medium or filter element of the present invention is placed upstream of an ion-exchange resin, it is sometimes referred to as a xe2x80x9cprefilterxe2x80x9d. When placed downstream of the ion-exchange resin, the medium or element is sometimes simply referred to as a xe2x80x9cfilterxe2x80x9d while the ion-exchange resin, coated on a mesh or other porous substrate, is termed a xe2x80x9cprecoatxe2x80x9d.
The present invention is also directed to an apparatus and to filter elements which are capable of reducing concentrations of contaminants to the above-indicated levels. The apparatus and filter element are also capable of removing fine solids ranging from submicron particle sizes (about 0.5 xcexcm) up to the largest particles found in aqueous particulate-containing liquids commonly encountered in the environment of a power plant. Typically, this is about 10 microns.